Real-time imaging is generating a wave of enthusiasm among professionals engaged in the assessment of fetal health by biophysical means. In this report the reproducibility and accuracy of biparietal diameters (BPD’s) obtained by real-time imaging are examined in depth. In addition, comparisons are made between BPD’s obtained by real time and by both gray-scale and B-scan sonography. It is shown that BPD’s obtained by real-time imaging correlate with true BPD’s of neonates (delivered by cesarean section) only if (1) the width of skull tables on Polaroid photographs are 3 to 5 mm. and (2) the measurements are obtained from the outer aspects (O-O) of the cephalic contour traced on Polaroid photographs. BPD’s measured from the outer to the inner (O-I) cephalic points are smaller than the true BPD’s of neonates by an average of 3.8 mm.—the difference is statistically significant, P